The present invention relates to a process and apparatus for improving the purification of vinyl aromatic compounds. More particularly, the present invention relates to a process and apparatus for the reduction in the formation of undesirable products in improving the utilization of polymerization inhibitor in the purification of vinyl aromatic compounds such as styrene, substituted styrene, divinylbenzene and polyvinylbenzenes. The improved utilization inhibitors result in a reduction in the amount of polymerized materials when the vinyl aromatic compounds are subjected to elevated temperatures such as in the distillation thereof.
It is well known that vinyl aromatic compounds such as monomeric styrene, lower alkylated styrene, e.g., alpha-methyl styrene, and divinylbenzene polymerize readily, and furthermore, that the rate of polymerization increases with increasing temperature. Inasmuch as styrene and divinylbenzene produced by common industrial methods contain impurities, these compounds must be subjected to separation and purification processes in order to obtain product suitable for most types of further industrial use. Such separation and purification is generally accomplished by distillation.
In order to prevent polymerization at the conditions necessary for the distillation of vinyl aromatic compounds, various types of known polymerization inhibitors have been employed in connection with distillation processes. For example, U.S. Pat. No. 4,469,558 to Watson shows a process for the purification of vinyl aromatic compounds and the utilization of dinitro-p-cresol (DNPC) as the polymerization inhibitor. U.S. Pat. No. 4,468,343 to Butler et al shows a process for the purification of vinyl aromatic compounds and the utilization of DNPC in combination with other components as the polymerization inhibitor. The inhibitor formulation is added to the various purification columns. U.S. Pat. No. 4,492,675 to Watson et al shows another process for the purification of vinyl aromatic compounds. The disclosures of these patents are hereby incorporated by reference in their entirety for purposes of U.S. prosecution. Other examples of inhibitors useful for inhibiting the polymerization of vinyl aromatics under distillation conditions include 4-tert-butylcatechol (TBC) and hydroquinone. It is preferred, however, to purify vinyl aromatics by using vacuum distillation techniques, whereby some of these processes and previously utilized inhibitors are rendered unsuitable in view of the fact that they are effective only in the presence of oxygen. Various inhibitors are well known in the art and range from sulfur to the more exotic and environmentally driven components. In a vinyl aromatic purification or distillation process, the residual material represents a significant pollution or waste removal problem.
However, the utilization of inhibitors must be controlled to minimize costs and in order to render these components useful in later processing. Most of these components are later subjected to controlled polymerization processes. For example, styrene is later utilized to make various polymers, including polystyrene. If the monomer is loaded with inhibitors, the polymerization process is adversely affected. Additionally, these additives add significant cost to the production process for vinyl aromatic compounds. Thus, there is a need to maximize the effect of any inhibitor that is utilized. In a typical distillation process for vinyl aromatic compounds utilizing a polymerization inhibitor, the mixture of vinyl aromatic to be distilled is generally contacted with the chemical polymerization inhibitor prior to being subjected to distillation conditions in the distillation apparatus. It remains a significant problem today that the amount of polymer formed in the distillation apparatus and in the high purity product recovered there from is substantially higher than desired, and occasionally, that complete polymerization occurs inside of the distillation apparatus.
The purification of vinyl aromatic compounds such as styrene is normally carried out in multiple zones or columns. This allows the user to stage the purification conditions in order to maximize the recovery of the desired product. As indicated in the above referenced patents, styrene purification at a styrene plant takes place in three or more columns operated in series. Each purification section or column is equipped with its own reboiler configuration. Each reboiler is normally a steam heat exchanger that provides the required heat for the purification process.
It is therefore desirable to provide new processes for the maximization of the effect of polymerization inhibitors in the purification of vinyl aromatic compounds.
It is therefore an object of the present invention to provide a method for maximizing the effect of polymerization inhibitors in the purification of readily polymerizable vinyl aromatic compounds.
A further object of the invention is to provide a new and improved process for the distillation of readily polymerizable vinyl aromatic compounds, which process results in higher recovery of a high purity unsaturated vinyl aromatic compound and concomitantly in the production of less undesirable by-products.
A further object of the invention resides in the provision of a new and improved process for the distillation of vinyl aromatic compounds which results in the production of substantially less polymerized material in the distillation apparatus and particularly in the reboiler sections.
It is also an object of the present invention to provide a new and improved process for the distillation of vinyl aromatic compounds, which permits the distillation apparatus to be operated at an increased rate of throughput without a reduction in efficiency.
It is still a further object of the present invention to provide a new and improved process for the distillation of vinyl aromatic compounds, which provides all of the foregoing enumerated advantages in a vacuum distillation process.
In accomplishing the foregoing and other objects, there has been provided in accordance with the present invention a process useful for the distillation of a readily polymerizable vinyl aromatic compound comprising adding a solution of inhibitor to the vapor phase section of the reboiler apparatus utilized as part of the distillation apparatus.
In one aspect of the process according to the invention, the inhibitor is introduced into the distillation system by adding it to the distillation columns. A side stream from the bottom of the distillation column is added to the vapor phase section of the reboiler used with the column. The stream is preferably introduced to the vapor phase section through at least one spray nozzle added at the top of the reboiler. The stream is taken from the bottom of the distillation column and preferably contains at least 50 ppm of inhibitor. The inhibitor level is controlled by the addition of inhibitor to the various points in the distillation apparatus other than the reboiler. The amount of inhibitor necessary to effectively inhibit polymerization of the vinyl aromatic compounds may vary over a wide range depending upon various factors of the distillation process, e.g., temperature, reflux ratio, pressure, residence time, etc. Typically, however, it has been found that an amount of the inhibitor between about 50 and about 3000 ppm is sufficient to inhibit polymerization of vinyl aromatic compounds under normal distillation conditions (105xc2x0 C., and above).
According to a further embodiment of the instant invention, also provided is a distillation method and apparatus for use with the purification of vinyl aromatic compounds. This method comprises introducing a feed of impure vinyl aromatic compound into a distillation apparatus; introducing an effective polymerization inhibiting amount of specified inhibitor into the distillation apparatus, and then distilling the feed under distillation conditions of elevated temperature through cycling the vinyl aromatic hydrocarbon through at least one reboiler (heat exchanger); and adding a side stream of the purification column bottoms to the top of the reboiler in the vapor phase section. This process allows the recovery of an overhead product of high purity vinyl aromatic product and a residual bottoms fraction having a reduced content of polymeric material.
In the preferred embodiment, the vinyl aromatic compound preferably comprises styrene, and is distilled in a distillation train comprising a benzene-toluene column, an ethylbenzene column, and a styrene column, although it is to be emphasized that the distillation method of the present invention is equally advantageous for use with other vinyl aromatic compounds and with other distillation equipment such as would be well known to those skilled in the art.
Through the use of the process according to the present invention, the amount of polymerization occurring within the distillation apparatus, and particularly within the reboiler, is significantly reduced in comparison to conventionally employed methods. In addition, the amount of desired distillation product is increased in proportion to the decrease in the amount of polymer formation.
Other objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments of the instant invention, taken in conjunction with the figures.